It is well known to connect two or more telephone sets, for example in different rooms of a residence, to the same two-wire telephone subscriber line, thus enabling two or more people in the residence to participate simultaneously in a telephone conversation via the telephone subscriber line. A disadvantage of such an arrangement is that the privacy of a person using a first telephone set may be lost due to another person taking a second telephone set, connected to the same telephone subscriber line, off-hook (lifting the handset). While in some circumstances a click may be audible to the person using the first telephone set when the second telephone is taken off-hook, this does not provide a convenient or reliable indication that privacy has been interrupted. Accordingly, it is desirable to provide a more reliable indication at one telephone set of the hook state of another telephone set connected to the same line.
Such an indication is also useful even when the first telephone set is on-hook, as it can then serve to indicate to a prospective user of this telephone set whether or not the telephone subscriber line is already in use by another person using one of the other telephone sets connected to this line.
Furthermore, it is desirable for a user of the first telephone set to be able to place this in a hold state during a telephone call, to go to a second telephone set connected to the same line to continue the call, and to have the first telephone set automatically release itself from the hold state when the second telephone set is taken off-hook. This is referred to as remote release from hold.
For each of these situations, a change in the voltage between the two wires of the telephone subscriber line can conceivably be detected and used to take an appropriate action, for example to provide an indication at the first telephone set or to effect a release of this telephone set from the hold state. However, it has been found to be very difficult to monitor the subscriber line voltage in an effective manner to accomplish this, because of the variability of this voltage and the presence of interfering signals on the line.
More particularly, the actual subscriber line voltage is dependent upon a number of variable factors, such as the resistance of the telephone subscriber line and the number and nature of the telephone sets connected to the line. Interfering signals on the line can occur due to hook switch flashing or pulse dialling activities at other telephone sets connected to the line, and due to short-duration open switch intervals at the central office to which the subscriber line is connected. Open switch intervals comprise temporary interruptions in voltage supplied from the central office to the subscriber line during switching activity at the central office, and known methods of remote release from hold are susceptible to incorrectly releasing from hold in response to open switch intervals.
An object of this invention, therefore, is to provide an improved method of and apparatus for detecting voltage changes on a telephone subscriber line.